“And Dauid sware moreouer, and said, Thy father certeinly knoweth that I haue found grace in thine eyes, and he sayth, Let not Ionathan know this, lest he be grieued: but truely, as the Lord liueth, and as thy soule liueth, there is but a step betweene me & death.”
1611 King James Version (KJV)
20:3 David sware - The matter being of great moment, and Jonathan doubting the truth of it, he confirms his word with an oath, which follows in the end of the verse. Only he interposeth a reason why Saul concealed it from Jonathan.